Syrtis Major and Hellas on Mars, illustration. Syrtis Major is a distinctive dark marking on the surface of Mars. It was initially believed to be a plain, but later data revealed a broad topographic rise. Its dark colour comes from the basaltic volcanic rock in the region and the relatively low amount of dust. In contrast, the lighter area is the Hellas impact basin, one of the largest impact craters in the solar system, measuring about 7,000 kilometres across.